Solution processing of organic materials, in particular semi conductors, offers great advantages for inexpensive, large-area, mechanically flexible applications. However, the conventional deposition methods and environmental sensitivity of most organic materials still make it challenging to achieve precise patterning of solution-processed films and to integrate different functional materials yielding well-defined features without material degradation.
The performance of light-emitting diodes (LEDs) and field-effect transistors (FETs) based on solution-processible organic semiconductors has improved rapidly in recent years and is now competitive with that of conventionally vacuum deposited small organic molecules, but also with that of established inorganic technologies. Both n-type and p-type organic FETs with mobilities comparable to that of amorphous silicon based devices have been demonstrated. Deposition and patterning of these materials by a combination of solution processing and direct write printing, such as inkjet, offset or flexographic printing, has been pursued as a new paradigm for electronic manufacturing for more than a decade promising to enable low-cost, large-area electronic devices on flexible substrates. However, for many applications the solution processibility of organic materials also imposes severe limitations on their use because at present the requirements for patterning resolution, reproducibility and yield cannot always be met by standard printing techniques.
Although approaches have been developed for high resolution patterning of organic semiconductors using techniques such as scanning probe microscopy, nanoimprinting, microcontact printing, advanced ink-jet printing, selective dewetting, phase separation, physical delamination, laser ablation, and transfer printing, conventional photolithography would for many applications be the technique of choice offering the highest level of reliability. However, soluble organic semiconductors tend to dissolve, or at least swell, in the solvents used for deposition of common photoresists resulting in severe degradation of electronic and optical properties. This is particularly problematic for device configurations which rely on the electronic properties of the top surface of the organic semiconductor. If, for example, one attempts to pattern the active semiconductor layer of a top-gate organic FET by photolithography prior to deposition of the gate dielectric, severe device degradation is observed. Moreover there are no good techniques available to clean the surface of an organic semiconductor after it becomes contaminated by photoresist, developer or solvent residues.
A further consequence of the fact that the solvents used for deposposition of common photoresists tend to dissolve or swell organic semiconductors is that it is even more challenging to pattern more than one semiconducting component in a single device without compromising the overall device performance. Particularly the formation of a well-defined functional lateral heterojunction between solution-processed organic materials has not been possible so far. This is a significant limitation in terms of the fabrication of advanced heterostructure device architectures, which would enable the combination of the unique properties of individual solution-processed semiconductors.
Hence there is a need for simple, versatile, high-resolution and clean patterning methods, which are applicable to a wide range of solution-processable organic materials and can be easily integrated into all common thin-film transistor (TFT) architectures without compromising device performance. Patterning methods enabling the fabrication of high-quality lateral heterojunctions are particularly desired.
It has now been surprisingly found that photolithography may be used in combination with a protective or sacrificial polymer layer to afford a patterning method that can be applied to all common TFT architectures as well as a wide range of solution-processable organic semiconductors. Advantageously the method facilitates patterning without device degradation and allows precise alignment of the semi conductor pattern with respect to previously defined electrodes and other substrate structures. The key to the process is use of a polymer, preferably a fluoropolymer, to protect regions of the semi conductor during photolithographic patterning.